One of the central results in mathematical relativity is the positive mass theorem which has first been proven by Schoen-Yau using minimal surfaces, and Witten using spinors. We show how the level-sets of spacetime harmonic functions can be used to give a new proof of the spacetime positive mass theorem. Moreover, we demonstrate how this technique leads to an optimal rigidity statement. This talk is based on joint work with Demetre Kazaras, Marcus Khuri and Yiyue Zhang.